1. Field of the Invention
The present invention relates to a sprocket wheel mounting unit for supporting a plurality of sprocket wheels of a bicycle, particularly for use with a rear wheel of the bicycle.
2. Description of the Prior Art
With an increased number of speeds provided by a derailleur of a bicycle today, a multistep sprocket wheel unit for the rear wheel includes an increased number of sprockets which is now 5 to 7. There are demands for a simplified mounting structure and easy mounting method. To meet such requirements, the multistep sprocket wheel unit for the rear wheel disclosed in Japanese Utility Model Publication Kokai No. 63-4893, for example, has a plurality of sprocket wheels in the form of ring plates arranged in order on a cylindrical boss with spacers in between. Each sprocket wheel is connected to an associated spacer by means of screws or the like. Splines are formed between the sprocket wheels and boss to prohibit relative rotation therebetween positively. Since this unit is used for the rear wheel of a bicycle, drive must be transmitted between the rear wheel axle and boss through a one-way mechanism. For this purpose, the boss is formed as an outer race of a one-way clutch, and the one-way clutch and inner race are disposed on an inner periphery of the boss.
In this multistep sprocket wheel unit, the teeth of the largest sprocket wheel are at a considerable distance to the boss, and this sprocket wheel needs sufficient thickness to avoid deformation at torque transmitting times. This results in an increased weight. If the boss is formed to have a large outside diameter in order to reduce the radial length of sprocket itself, the boss must have an increased thickness since the rear wheel axle has a standardized size, which again results in an increased weight.
Further, for using spacers to adjust mounting pitches of the sprocket wheels, a plurality of spacers must be manufactured with high precision. Assembly work also becomes troublesome with an increase in the number of sprocket wheels.